CHONDROPROTECTION THROUGH CYTOPROTECTION The approaches to chondroprotection have primarily targeted modifications in the extracellular matrix remodeling process. By contrast, only minor attention has been given to the role of chondrocyte death and survival in cartilage destruction. The present application will address the notion that inhibition of chondrocyte apoptosis represents a new target for chondroprotection. Previous work performed in this laboratory identified chondrocyte apoptosis as an important pathogenetic determinant in both human and experimentally induced osteoarthritis (OA). Chondrocyte apoptosis is a regular feature of arthritic cartilage and correlates with the degree of cartilage matrix destruction. Chondrocyte apoptosis has also been linked to the abnormal calcification of articular cartilage and meniscus extracellular matrix. Chondrocyte death leads to the depletion of the cells that are responsible for cartilage homeostasis and remodeling. Furthermore, cartilage does not contain phagocytes and remnants of dead cells such as apoptotic bodies which remain in the tissue and can lead to matrix calcification and degradation. We also showed that mechanical cartilage injury induced chondrocyte apoptosis. Our preliminary studies showed that caspase inhibitors reduced cell death and attenuated matrix loss in vitro and in vivo. Based on these findings we propose the following hypothesis: Inhibition of post-traumatic chondrocyte death will reduce cartilage loss and support tissue repair, thereby reducing the risk for the development of OA. The project has two major components, an in vitro study on mechanisms of chondrocyte apoptosis induced by mechanical stress (Specific Aims 1- 3), an animal models of joint injury to assess the efficacy of pharmacologic inhibition of chondrocyte apoptosis (Specific Aims 4 and 5) (1) Study the expression of death receptors and their ligands, and the intracellular apoptosis regulators Bcl-2 and Bax. (2) Analyze the role of mitochondria and nitric oxide in chondrocyte apoptosis and establish the profile of caspases that are activated after mechanical cartilage injury. (3) Inhibit apoptosis in mechanical cartilage injury in vitro and determine the long-term consequences on extracellular matrix degradation and synthesis. (4) Monitor the occurrence of chondrocyte apoptosis and determine the implications for cartilage degradation in an animal model of joint trauma. (5) Evaluate chondroprotective effects of apoptosis inhibitors in rabbits with anterior cruciate ligament transection, a model of cartilage injury and osteoarthritis.